An injector for a vehicle engine is driven by a solenoid valve and sprays fuel for a predetermined period of time to produce a fuel-air mixture. A common-rail fuel injection system disclosed in JP-A-11-210533 is used for a diesel engine and includes a break detection circuit for detecting a break in an injector. In the fuel injection system, flyback energy generated in a solenoid coil in a control valve for controlling discharge rate of a supply pump charges a capacitor for driving a solenoid valve of the injector. The break in the injector is detected based on the capacitor charge condition at a predetermined time.
A driving circuit disclosed in JP-A-10-9027 includes a circuit for passing a subtle current through a solenoid and a resistor connected in series with the solenoid during a period when an injector is not driven. The subtle current is small enough not to drive the solenoid. A break in the injector is detected by comparing the subtle current flowing through the resistor with a reference current.
A fuel injection electronic control unit (ECU) increases a flyback voltage (i.e., back electromotive force), which is generated when a solenoid valve is turned off, to about 70 volts to improve the responsivity of the solenoid valve of an injector. Therefore, a driver integrated circuit (IC) used in the fuel injection ECU is required to withstand a voltage greater than the flyback voltage of about 70 volts.
For example, a solenoid valve of an injector is broken down when a solenoid coil is broken (i.e., open-circuited) or short-circuited to a ground line, or a power line. The present inventor considers adding a diagnosis function to detect a break in the solenoid coil to such a driver IC. A driver IC 1 shown in FIG. 7 includes a MOSFET 3 for energizing a solenoid coil 2 as a load, a gate driver 4 for driving the MOSFET 3, and a break detection circuit 7 for implementing a diagnostic function to detect a break in the solenoid coil 2. The break detection circuit 7 includes a target voltage generator 5 and a window comparator 6.
When the solenoid coil 2 is broken (i.e., the solenoid coil 2 is open-circuited), the target voltage generator 5 exhibits a voltage clamp action. The voltage clamp action clamps a voltage at an output terminal 1d to a target voltage VT during a period of time when the MOSFET 3 is off. The target voltage generator 5 has less ability to output an electric current. Therefore, when the solenoid coil 2 is not broken, the target voltage generator 5 does not exhibit the voltage clamp action. The break detection circuit 7 compares the voltage at the output terminal 1d with a reference voltage by using the window comparator 6 and outputs a break detection signal Sdiag based on a result of the comparison.
In the driver IC 1, a flyback voltage of about 70 volts is applied to the output terminal 1d. Therefore, the driver IC 1 is required to withstand a voltage greater than the flyback voltage of about 70 volts.